1. Field of the Invention
The present invention relates to a motor control apparatus which converts alternating current (AC) power supplied from a three-phase AC input side into direct current (DC) power to output to a DC link, and then further converts the DC power into AC power for driving a motor, and in particular relates to the motor control apparatus including a protection operation command unit which commands the motor to perform a predetermined protection operation at the time of power failure on the three-phase AC input side.
2. Description of the Related Art
In motor control apparatuses for driving motors in machine tools, forging machines, injection molding machines, industrial machines, or various robots, AC power supplied from a three-phase AC input side is once converted into DC power and then further converted into the AC power to be used as drive power for a motor provided for each driving axis. Motor control apparatuses include a rectifier which rectifies AC power supplied from the three-phase AC input side and outputs DC power and an inverter which is connected to a DC link on a DC side of the rectifier and mutually converts the power between the DC power of the DC link and the AC power which is drive power or regenerative power of the motor, and they control a speed, torque, or a position of a rotor of the motor connected to an AC side of the inverter.
In such a motor control apparatus, when power failure occurs on the three-phase AC input side of the rectifier and a three-phase AC input voltage is decreased, the motor is not able to continue a normal operation. Thus, a failure may occur such as breakage and deformation of the motor, the motor control apparatus for driving the motor, a tool connected to the motor driven by the motor control apparatus, a machining target machined by the tool, a manufacturing line including the motor control apparatus, and the like. Therefore, a power failure detection unit is provided on the three-phase AC input side of the rectifier to monitor whether power failure occurs or not on the three-phase AC input side of the rectifier, and when the power failure detection unit determines that the power failure has occurred on the three-phase AC input side of the rectifier, the motor control apparatus performs a protection operation to avoid or minimize the above-described failure.
As a power failure detection method, for example, there is a method as described in Japanese Unexamined Patent Publication No. 2006-14546 in which a three-phase AC input voltage of an AC power source side of a rectifier is converted into an equivalent voltage vector in a two-phase coordinate by performing coordinate transformation, a power source voltage amplitude value is calculated by calculating an amplitude of the vector, and power failure is detected when a state in which the amplitude value is below a predetermined reference voltage value continues for a predetermined reference time length.
FIG. 6 illustrates the power failure detection method according to the invention described in Japanese Unexamined Patent Publication No. 2006-14546. A motor control apparatus 101 includes a rectifier 111 which rectifies AC power supplied from a three-phase AC input power source 103 and outputs DC power and an inverter 112 which is connected to a DC link as a DC side of the rectifier 111 and converts the DC power output from the rectifier 111 into AC power of a desired voltage or a desired frequency to be supplied as drive power of a motor 102 or converts the AC power regenerated from the motor 102 into the DC power, and the apparatus controls a speed, torque, or a position of a rotor of the motor 102 connected to the AC side of the inverter 112. An AC reactor 121 is disposed on the three-phase AC input side of the rectifier 111, and a smoothing capacitor 122 is disposed on the DC link which is on the DC output side of the rectifier 111. A voltage amplitude calculation unit 114 calculates a power source voltage amplitude value from an AC voltage value on the three-phase AC input side of the rectifier 111 detected by an AC voltage detection unit 113. A power failure detection unit 115 determines that power failure has occurred on the AC input side of the rectifier 111 when a state in which the power source voltage amplitude value is below a predetermined specified voltage value continues for a specified time. When the power failure detection unit 115 detects power failure, the inverter 112 is notified of a protection operation start command.
A protection operation to be performed when power failure occurs has an advantage that can protect the motor, the motor control apparatus, a tool, a machining target, a manufacturing line including the motor control apparatus, and the like, however, once the protection operation is executed, the manufacturing line is stopped which causes an economic loss in reality. It is not preferable to perform the protection operation in a case of power failure which occurs suddenly, does not last long, and is recovered immediately (hereinbelow, referred to as “instantaneous power failure”). Thus, the motor control apparatus and peripheral devices thereof (e.g. a control power source unit and a coolant device) are each provided with an energy storage unit such as a capacitor so as to be able to continue a normal operation for a certain period of time using energy stored therein when an instantaneous power failure occurs, and execution of the protection operation is suppressed to a necessary minimum. An energy amount enabling the motor control apparatus and the peripheral devices thereof to continue the normal operations when the instantaneous power failure occurs is referred to as “instantaneous power failure tolerance”. After recovery from the instantaneous power failure, it is necessary to store the above-described energy for continuing the normal operation again in the energy storage unit to prepare for future power failure. The above-described specified voltage value and specified time used as power failure determination conditions of the power failure detection unit are generally determined in response to the instantaneous power failure tolerance. Energy for performing the protection operation at the time of power failure is less than the energy enabling the motor control apparatus and the peripheral devices thereof to continue the normal operation when the instantaneous power failure occurs and also supplied from the energy storage unit.
When the power failure on the three-phase AC input side of the rectifier (i.e., the AC power source side on which the three-phase AC input power source exists) is a single instantaneous power failure, energy can be sufficiently stored in the energy storage unit up to the instantaneous power failure tolerance enabling the motor control apparatus and the peripheral devices thereof to continue the normal operation. However, when the instantaneous power failure successively occurs, it is not able to sufficiently store the energy up to the instantaneous power failure tolerance, and the motor control apparatus and the peripheral devices thereof are not able to continue the normal operation itself. In such a case, when the power failure is detected thereafter, the protection operation is not able to be executed because of lack of energy, and accordingly, a failure may occur such as breakage and deformation of the motor, a motor control apparatus for driving the motor, a tool connected to the motor driven by the motor control apparatus, a machining target machined by the tool, a manufacturing line including the motor control apparatus, and the like.
If power failure detection sensitivity of the power failure detection unit is increased so that the protection operation is inevitably executed when the instantaneous power failure occurs regardless of single or successive, occurrence of the above-described failure such as breakage and deformation can be avoided, but an economic loss may occur due to stop of the manufacturing line as a result of execution of the protection operation, which is not efficient.